From Brauwelt, Vol. 4, pages 118-125 (1987) and Filtration and Separation, pages 198-200 (1989), it is known for the recovery of beer from the liquid product obtained by the fermentation and which may contain surplus yeast, to carry out a filtration of the turbid liquid under pressure through at least one microfiltration module utilizing the tangential mode of filtration.
In the tangential filtration mode, the suspension to be filtered is passed across the porous filtration surface in a substantially continuous flow and under pressure so that some of the liquid will traverse the filtration membrane and can be collected on the opposite side thereof as a filtrate or permeate. The balance of the liquid and the solids of the suspension which cannot pass through the microporous membrane, continue along the surfaces of the filtration module as a retentate.
The process operates substantially continuously with recirculation of the retentate stream to the microfiltration module. The suspension to be filtered is fed continuously to the retentate stream upstream of the module and a concentrate stream with a substantially constant turbidity or suspended-solids concentration is continuously withdrawn from the retentate stream downstream of the module. When the process is used for the recovery of beer from the turbid liquid containing the surplus yeast, the yeast constitutes the turbidity and the clear or bright recovered liquid is the beer.
In the earlier systems described, tube bundle modules are generally employed and are traversed by the suspension to be filtered in turbulent flow. The filter tubes are constituted by tubular microporous ceramic membranes. The microfiltration unit can be equipped with a heat exchanger for cooling the retentate stream. The filtrate pipes for the microfiltration unit can be matched thereto by providing a plurality of microfiltration modules in cascade or series in each of a plurality of parallel segments with the filtrate pipes being connected to the modules of these segments (see German Patent Document DE-OS 37 00 804).
In the process described in which the suspension to be filtered is continuously fed to the circulated retentate stream and a concentrate stream is continuously withdrawn, it is problematical to continuously or repeatedly measure the turbidity concentration in the retentate stream for controlling the process. In general, turbidity measurements and specific gravity measurements must be constantly taken by measuring processes which are complex, expensive and prone to error, i.e. are not reliable.